Method and apparatus for adjusting radio echo detection systems



May 8, 1951 J. L. LAWSON METHOD AND APPARATUS FOR ADJUSTING RADIO ECHO DETECTION svs'mms Filed June 24, 1943 G a u 2 v 3 (Q 2 mm 8 F 2 6 l. O R n w M m E F mi N m a e 1 R f u 2 w R w w a m H E I l R IMUIHU 4 v 2 6 3 3 l kl A T MITTER 40--TRANSMITTER I. 50-RECEIVER FIG.3

INVENTOR JAMES L. LAWSON ATTORNEY Patented May 8, 1951 METHOD AND APPARATUS FOR ADJUSTING RADIO ECHO DETECTION SYSTEMS James L. Lawson, Ann Arbor, Mich., assignor, by mesne. assignments, to the United States of America as represented by the Secretary of the Navy Application June 24, 1943, Serial No. 492,062

Claims.

This invention relates to signalling, locating and detection systems employin high-frequency electromagnetic waves in which a transmitting device and a receiving device are used with a common antenna. More particularly, this invention concerns arrangements in systems of the type referred to for the purpose of reducing loss of signal energy in that part of such a system which includes the transmitter.

It is an object of this invention to provide a method whereby the absorption of signals which are desired to be received, resulting from signal energy going into the transmitter part of the system, may be inhibited and minimized, thereby increasing the signal strength at the receiver.

I have found that transmitting tubes, such, for instance, as multi-cavity magnetrons, when quiescent may present an impedance to the transmission lines to which they are normally coupled which is quite different from the impedance exhibited during periods when oscillations are being generated in the transmitting tube. Thus, although the transmitter is normally matched to the line for maximum energy transfer during periods of activity, during periods of quiescence it will be mismatched to the line and reflections will occur. By my method I utilize these reflections in such a way that a minimum of energy is accepted by the line connecting the transmitter to the junction of the receiver and transmitter lines.

According to this invention the foregoing result is obtained by adjusting the length of the line connecting the transmitter and the aforesaid junction.

The invention may be better understood with reference to the accompanying drawings in which:

Fig. 1 shows one form of apparatus with which this invention may be applied in order to improve 1the performance of a high-frequency radio sys- Fig. 2 shows another arrangement for applying this invention to high-frequency radio systems, and

Fig. 3 shows still another arrangement for applying this invention to a high-frequency radio system.

Referring to Fig. 1, there is shown at the left a transmitter which includes a multi-cavity magnetron for generating high-frequency electromagnetic oscillations, which magnetron is diagrammatically shown at Ill. The transmitting tube H! is matched to the coaxial transmission line H by a suitably designed matching transformer |2 which consists of a quarter wave section of line with the relative radii of the inner and outer conductors differing from the corresponding dimensions of the transmission line H in such a manner as to produce matching of impedances under conditions of transmitter opera tion, in accordance with principles which are well understood in the art. The coaxial transmission line H includes an outer conductor l3 and an inner conductor M. The coaxial transmission line H leads to a section of coaxial transmission line which may be described as a trombone section, because said section of coaxial transmission line is connected by sliding contacts at its end so that the total length of the transmission line of which it forms a part may be varied and adjusted in any desired position. The so-called trombone section is shown at l5 and the two sliding connections are shown at l6 and II. It is to be noted that the sliding connections must provide for a good contact between the inner conductors as well as good contact between the outer conductors. From the trombone section If) the coaxial transmission line is continued by a further length of coaxial transmission line shown at I8 having an inner conductor l9 and an outer conductor 20. At the point 2| there is a T-J'unction formed by the coaxial transmission line I8 and another coaxial transmission line 22, the inner conductor of which connects with the conductor l9 and the outer conductor of which connects with the conductor 20. The coaxial transmission line l8 leads beyond the junction 2| to an antenna 23, which is usually provided in the customary fashion with a matching transformer 24, a parabolic reflector 25, an auxiliary reflector 25a and various other mechanical and electrical. arrangements not shown and havin no bearing on this particular invention. The transmission line 22 leads to an automatic electrical breakdown device designed to short-circuit this transmission line while the transmitting tube I0 is active. This device is shown at 26 and is of the type commonly known as a resonant transmit-receive switch or T-R box and is more fully described in one of my copendingapplications; Serial No. 479,662, filed March 18, 1943 and enttiled, Protection of Receiver Against Overload. From the T-R switch 26 a transmission line 21 leads to a receiver 28. a

When the length of the line between the transmitting tube l0 and the junction 2| has certain values, the length of the line and the impedance of the quiescent'transmitting tube will combine to present a very high impedance at the junction 2| during periods when the transmitting tube is quiescent. For other lengths of the line between the transmitting tube Ill and the junction 2| a lower impedance will be presented at the junction 2! which will cause a portion of the energy of any received signal from being absorbed in the transmitting tube and the associated transmission line,

presented at the junction 2| and a considerable portion of the energy of received signals will be absorbed in the transmitting tube and its associated line.

I have found that the signal strength can be maintained practically unimpaired by proper ad.- justment of the line length. I:find :further that the maxima of signal absorption by the transmitter system are much sharper with respect to changes in line length than are the .minima of signal absorption. Consequently my preferred method for adjusting the line length for maximum received signal consists in first adjusting the line length (which in the apparatus shown in ,Fig. 1 would be accomplished -by moving the trombone section I75 ac a d f rthoni s sliding c ntacts. until. a minimum signal is obtained in the receiver 28 and then by adding or subtracting one.- quarter wave length in the effective length of the line between the transmitter l and the junction 2! by a further movement of thetrombone section I5, In this fashion not only is it assured that the highest impedance will be presened by the transmitter line at the junction 2! to a Si al of the desired frequency (whereas the adjustment would be less certain ,if it was attempted simply to adjust for maximum received signal in the receiver, the maximumnotbeing .very sharp with respect to variation in the line length) but also an adjustment is .obtained which is as far as possible from the undesired condition of maximum absorption in the transmitterline, s that the systems is given the best possible immunity against loss of reception upon small changes in fr qu ncy or sm l changes in l n length resulting from thermal expansion and the like.

Fig. 2 shows asystem similar to that of F g. 1

employing a different method for varying line lengths in which the arrangement has the ,disadvantage of failing to present continuous variability but has sQme advantages of convenience in practice and the achievement of better electrical contacts than are usually obtainable with sliding joints. Theapparatuslof Fig. 2 again con- ,tains ,a transmitter with a transmitting tube Ill, matching transformer l2, coaxial lines H, 18, 22 and .21. a junction 2|, an .antenna 23, a T.R .switchifiand a receiver 28.

Instead-of the trombone section 15, one or severalofa numberof short sections of .coaxial'line, which may be called ?spacers,-" are inserted in the line, as thesection ,39 in Fig. 2 is inserted between the l-ine -l l and the line 18. Suitable joint contact arrangements are provided at the points of contact indicated on 2 at 5!. Since these contacts do not have to permit a continuously variable sliding movement, spring pressure and :frictional engagement may :be used to good advantage for maintaining electrical contact as is common in many types of electrical connectors. If desired, the contact of the outer conductor maybe further tightenedby a threaded clinching ring (not shown). The method for applying this :in'vention to the system shown in Fig. 2 is essentially the same as that previously described in connection with Fig. 1. Although the desired results-may sometimes be obtained by'simply-trying receiver response and then either adding or subf-tracting one-quarter wavelength (of the ,oscillations in the line) respectively to or from the total length .of spacers previously determined.

In cases where it is possible to provide replacement transmitting tubes sufiiciently similar to the tube to be replaced to permit dispensing with the'readjustmentofthe line length when one tube is substituted for another, the desired line length, in accordance with the present invention, between the transmitting tube and the junction 2! may be determined as herein described and then incorporated into the design of an apparatus with nonadjustable lines. The elimination of sliding .or separable joints thereby achieved has advantages of simplicity and of transmitting efliciency.

The advantages of this invention may also be realized by a similar procedure in systems which employ hollow pipe wave guides instead of coaxial transmission lines for the transmission of energy in the system. In the case of wave guides of the hollow pipe variety a great man methods are available for varying the efiective electrical {length of the wave guide system in addition to trombone sections and methods involving the insertion of spacing lengths. One method consists in simply cutting a slot in the wall of a hollow pipe wave guide. The slot is cut longitudinally of the wave guide, preferably for a distance of several wave lengths. Mechanical arrangements are then provided for squeezing the wave guide together in the neighborhood of theslot. Variation of the width of the wave guide in this fashion causes the Wave length of the oscillations in the wave guide to vary, so that although the physical length of the wave guide remains the same, the electrical length calculated in wave lengths may be made to vary quite considerablyv Other methods of varying the electrical length of a wave guide include methods involving the moving of masses of dielectric material within the wave guide in such a fashion as to alter the velocity of propagation and consequently also the wave length.

Fig. 3 illustrates the application of the invention in the manner just mentioned to a system in which hollow pipe wave guides are used for transfer of energy from one part of theSy-stem to another and in which, as above mentioned, the effective electrical length of the wave guide between the transmitter and the transmitterreceiver junction is varied by squeezing a slotted section of the wave guide. Referring to Fig. 3, the transmitter is shown generally at M and in cludes a inulti-cavity magnetron tube it which is coupled by a short section of coaxial line 42 to a rectangular hollow pipe wave guide 43. The wave guide 43 is also connected with an antenna system including an antenna 4d, a reflector dipole I35, and a parabolic reflector 45. In order to simplify the illustration, arrangements for changing the orientation of the antenna system are omitted in the drawing, although they are frequently provided in apparatus of this class. The end of the wave guide d3 adjacent to the antenna M is of course open.

Another wave guide 41 forms a junction atone of its ends with the wave guide 33 and at the other end is connected to an automatic electric breakdown device 48. The latter device is connected b means of a wave guide 39 to a receiver shown generally at 50.

The wave guide 43, between the end which is coupled to the transmitter and the junction with the wave guide 41 is provided witha longitudinal slot 5|. The slot 5| should be cut in one of the walls of the Wave guide 43 which is perpendicular to the direction of the electric vector of the oscillations in the wave guide so that an electric field will not appear across the slot and propagations along the wave guide of the oscillatory energy in question will not :be appreciably impeded. A clamping device 52 provided with a screw thread adjustment 53 is mounted on the wave guide 43 in the neighborhood of the slot in such a way that the width of the slot 5! may thereby be adjusted. In the form of device shown in Fig. 3 the resilience of the metallic walls of the wave guide 63 are relied on for expanding the slot 5] upon the removal of clamping pressure previously applied. Other types of devices could of course be substituted for regulating the width of the slot 51 in a double ,acting fashion which would be independent of the resilience of the wave guide wall. As previously mentioned, the variation of the width of the slot 5| is accompanied by a rariation of the width of the wave guide 43, which in turn causes a change in the effective electrical length of the wave guide 43 in terms of wave length. In order to carry out the invention in an apparatus arranged such as that of Fig. 3, according to the preferred method of adjustment, two different settings of the screw thread adjustment 53 of the clamp 52 are first found at which a minimum received signal is observed by the receiver 50 when a given signal is introduced into the system. These two settings of the adjusting device 53 should be consecutive minimum signal settings, that is, there should be no minimum signal setting between them. After such minimum settings have been determined, the device 53 is then adjusted to a mean position intermediate of the two minimum signal positions, which will then represent an adjustment of the electrical line length of the wave guide 43 such that absorption by the transmitter and its line of the received signal is at a minimum and the received signal is at a maximum, and consequently the wave guide 53 is then adjusted so that a relatively large change in line length (of the order of one-quarter wave length, that is) will be necessary to cause appreciable absorption of the incoming signal in the transmitter and its associated wave guide system,

'minimizing the effect of the thermal expansion of the transmitter line.

In the foregoing description of the arrangements for carrying out the present invention, the experimental technique of determining the desired adjustment of line length has been described without adverting to the theoretically desired line length. In practice, the experimental method described is generally more useful, especially where the line between the resonant cavities of the magnetron transmitting tube and the junc tion of the transmitter and receiver feed lines may be composed of sections of transmission line or hollow pipe wave guide of different characteristics. In apparatus where the design research has proceeded so far that the length of the transmission line can be adjusted in manufacture to a V desired electrical length, theoretical calculations of the desired line lengths may sometimes supplement the experimental procedures outlined above.

A transmitting tube, such as a multi-cavity magnetron, when quiescent presents an impedance at a given point, such as at the loop which couples to the oscillating field of the tube, which impedance is considerably different than the impedance presented as such point during transmitter operation. The difference of impedance may be due to a number of causes, such as (1) a difference between the resonant frequency when the transmitter is in operation and the resonant frequency of the tube structure in the absence of direct current anode voltage and, possibly, (2) resistive loading during operation of the transmitting tube.

Although no general rules are applicable to all multi-cavity magnetrons with respect to the impedance change between transmitting conditions and quiescent conditions, for certain fixed types of magnetron tubes as designed for operation within certain wave length limits, this impedance change, when referred to a particular point in the apparatus, may be satisfactorily predicted on the basis of measurement of samples. In such case the desired electrical length between the said point of reference and the junction of the transmission means leading to the receiver with that leading from the transmitter to the antenna can be reasonably well determined by calculation. If at such point of reference the impedance of the transmitting tube is considerably lower when quiescent than when energized, the electrical distance from such point to a junction such as the junction 2| should be an odd number of quarterwave lengths. The electrical distance from such point to a junction such as that formed by the wave guides 43 and 47, should, however, be an integral number of half-wave lengths in the aforementioned case. This difference between the desired lengths for coaxial conductor and hollow pipe wave guide systems respectively lies in the fact that total reflection at the junction from the transmitter branch in the coaxial conductor system is obtained by producing a high impedance at the junction, whereas in the hollow pipe wave guide case the desired total reflection at the junction is obtained by having the transmitter branch of the system exhibit a low impedance at the junction.

In a case in which the transmitting tube shows a cold impedance considerably greater than the hot impedance, at the point of reference in question, the situation is reversed and the desired e distance from such point to the junction would be an integral number of half-wave lengths for the coaxial conductor case and an odd number of quarter-wave lengths for the hollow pipe wave guide case. If at the point of reference no considerable difference between hot and cold impedance of the tube appears, this signifies either that another point of reference should be chosen or that the invention is not applicable to that particular variety of transmitting tube.

Another method of utilizing the variation of the length of the output line connected to a magnetron type of transmitting tube in order to prevent absorption of energyby the transmitting tube when the latter is quiescent relates to systems in which two magnetron transmitter tubes are employed to provide alternate single pulses to'a common radiating system. In such a system there will be a junction of the two out put lines ofthe respective transmitting tubes. The principle of this invention would then be applied for adjusting the length of the output lines of one of the magnetrons so that the total length of line between the two magnetrons is such that one will not appreciably absorb power While the other is operating. The danger of damage to the magnetron during operation of the other may, however, seriously limit the utility of such an arrangement. Such a system would not, moreover, be practical unless two magnetrons operating on exactly the same wave lengths were used and :in addition it would :be necessary to provide some means other than that described above for preventing either of the magnetrons from absorbing energy from the signal which it is desired to obtain in the receiver. Other means not covered in this invention are practical in certain mes of installationsfor instance a ChOkG afil a resonator acting as a choke could be associated with the transmission line leading from the transmitting tubes to the junction of the transmitter and receiver lines, which choke would be adapted to be short circuited by breakdown of a discharge gap during periods of transmission.

Prior to this invention, it was difiicult, if not impossible, to adjust radio signalling apparatus using a transmitter, a receiver, and a common a11- tenna system so that the received strength would not drop oi'i upon extremely slight changes (such as thermal effects) which would cause much of the received signal energy to be absorbed in the transmitter part of the system. If by some piece of good luck an arrangement might have been put together in which adequate received signal strength was obtained under some conditions there was no assurance that the signal strength would not drop ofi suddenly due to effects then unknown, but now known to be due to the line length between the transmitter and the junction 2! being such that only an extremely small change in its length, such as might be caused by thermal effects, would bring the transmitter branch of the line to a length corresponding to maximum absorption of energy in that line.

What I desire to claim and secure by Letters Patent is:

1. In a radio echo detection system including a transmitter, an antenna, a receiver, first wave energy transmission means connecting said transmitter to said antenna, and second wave energy transmission means connected to said receiver and forming a junction with said first transmission means, the method of sharply adjusting said system to minimize loss of received echo signals to said receiver which consists in directing said antenna at a remote target, operating said transmitter to launch energy toward said target, adjusting the electrical length of that portion of said first wave transmission means between said transmitter and said junction for minimum echo signal reception in said receiver, and then chang 'ing the electrical length of said portion by a quarter wave length of the waves transmitted in said system.

2. In a radio echo detection system including a transmitter, a receiver, a common antenna, first wave energy transmission means connecting said transmitter to said antenna, and second wave energy transmission means connected to said receiver and forming a junction with said first transmission means, the method of sharply adjusting said system to minimize loss of received echo signal to said receiver which consists in introducing a signal into said system while said transmitter is quiescent, adjusting the electrical length of that portion of said first wave transmission means between said transmitter and said junction for maximum energy absorption in said transmitter, and then changing the electrical length of said portion by a quarter wave length of the waves transmitted in said system.

3. In a radio echo detection system including a transmitter, a receiver, a common antenna, a

first hollow pipe wave guide connecting said transmitter to said antenna, and a second hollow pipe wave guide connected to said receiver and forming a junction with said first wave guide, the method of sharply adjusting the system to minimize the loss of received echo signals to said re. ceiver which consists in introducing a signal into said system, modifying a dimension of that portion of said first wave guide between said transmitter and said junction to obtain a first minimum received signal in said receiver, further modifying said dimension to obtain a second minimum received signal in said receiver, which second minimum is consecutive to said first minimum, and then modifying said dimension to the mean intermediate value of the two minimum signal values.

4. In a radio echo detection system including a transmitter, a receiver, a common antenna, first wave energy transmission means connecting said transmitter to said antenna, and second wave energy transmission means connected to said receiver and forming a junction with. said first transmission means, the method of sharply adjusting said system to minimize the loss of received echo signals to said receiver which consists in introducing a signal of the frequency of said transmitter into said system, mechanically modifying that portion of said first wave transmission means between said transmitter and said junction, thereby modifying the effective electrical length of said portion to obtain minimum signal reception in said receiver, and then further mechanically modifying said portion of said first transmission means to change the eifective electrical length thereof by a quarter wave length of said signal.

5. In a radio echo detection system including a transmitter, an antenna, a receiver, first wave energy transmission means connecting said transmitter to said antenna, and second wave energy transmission means connected to said receiver and forming a junction with said first transmission means, the method of tuning said system which consists in directing energy from said system toward a remote target, adjusting the electrical length of that portion of said first transmission means between said transmitter and said junction for minimum echo signal reception in said receiver, and then changing the electrical length of said portion by a quarter wave length of the waves transmitted in said system.

JAMES L. LAWSON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 1,927,393 Darbord Sept. 19, 1933 2,036,164 Usselman Mar. 31, 1936 2,129,669 Bowen Sept. 13, 1938 2,213,104 Gluyas, Jr Aug. 27, 1940 2,281,274 Dallenbach et al. Apr. 28, 1942 2,301,160 Finch Nov. 3, 1942 2,400,796 Watts et a1 May 21, 1946 2,401,717 Wolff et al. June 4, 1946 2,424,156 Espley July 15, 1947 2,439,656 Hausz Apr. 13, 1948 FOREIGN PATENTS Number Country Date 358,917 Great Britain Oct. 14, 193 1 108,349 Australia,, Sept. 15, 193.7 

